High Cohesion (HC)

Variants and Alternative Names

Context

Principle Statement

Description

The cohesion of a module is a measure for how well the internal parts of a module (e.g. the methods and attributes of a class) belong together. Having a high cohesion means, that a module should only comprise responsibilities which belong together.

Several kinds of cohesion can be distinguished some of which are strong and some of which are loose. So strong forms of coupling should be preferred: add explanation of cohesion types

Rationale

Not adhering to this principle, i.e. having a low cohesion, means that one module has several unrelated or only loosely related responsibilities. A change in the requirements for one of these may thus also affect the others which would not be the case in a highly cohesive module.

Strategies

Divide one large module into several smaller but more cohesive ones

Caveats

Origin

W. P. Stevens, G. J. Myers, L. L. Constantine: Structured design

Evidence

Examined: There are metrics that try to measure cohesion and there are studies relating these cohesion measures to the number of errors found during testing 1). This correlation is evident. The limitation of these studies is that these cohesion metrics cannot represent the cohesion notion completely.

Accepted The concept of high cohesion is widely known and described in several well-known books for example in

Contrary Principles

More Is More Complex (MIMC): Making a module highly cohesive often results in additional modules. Sometimes it is simpler to assign a minor unrelated responsibility to a module, which lowers the cohesion.

Model Principle (MP): Adhering to HC sometimes means to split up a class into several smaller ones which might correspond to the model less well.

Low Coupling (LC): A system consisting of one single module has a very low coupling as there are no dependencies on other modules. But such a system also has low cohesion. The other extreme, very many highly cohesive modules, naturally has a higher coupling between the modules. So here a compromise has to be found.